1. Technical Field
The present invention relates to a rotatable valve with housing made in two halves and assembled over the valve member/control element, all the surfaces coming in contact with the fluid being lined with a corrosion resistant plastic.
The valves of the above type are generally used in chemical/pharmaceutical plants handling corrosive chemicals. The purpose of the valve being controlling or shutting-off the flow of fluids through pipelines in the plant. The valves are operated using either a handle/lever fixed on top of the stem or with an actuator fitted over the top portion of the valve by means of a bracket/flange.
2. Background Art
A typical valve arrangement includes a valve housing comprising a body defining a central chamber, and having flow ports at the two axial ends thereof, and a shut-off or control element, which is generally a ball having a through bore with which a shaft is rotatably coupled. The body housing is generally made in two halves, side split or center split, being assembled together with suitable fasteners such as bolts or studs. Shut-off of fluid is achieved with the help of sealing elements or seat-rings made of plastic or elastic material, which are associated with the said housing/body, encircling the said body fluid ports and pressing over the said ball. The passage of the shaft through the housing being sealed by a second sealing element/packing, which is disposed in an annular space between the shaft and the housing. The packing can be compressed by a cover/bushing which is mounted axially along the shaft. The valve arrangement is provided in the interior of the housing with a lining of corrosion resistant plastic material like PTFE, PFA, FEP, PVDF, ETFE etc which extends at least up to the packing, with which it is in direct contact. The lining of the housing also extends over the joining face between the two body 10) halves and acts as a sealing gasket.
The U.S. Pat. No. 5,979,491 (Xomox) has an arrangement of a side split body housing, with a ball and shaft/stem made as separate parts to facilitate assembly of the valve. The stem fits into a socket or recess made in the ball. In this arrangement repeated operations of the valve can put stress on the soft plastic lining covering the ball and stem in the socket area. This can create a play between the ball and stem, due to wear/cold flow of the plastic lining over a period of time. The play makes accurate control and positioning of the ball impossible. Excessive wear can also lead to failure/cracking of the lining and cause corrosion of the metal insert of the stem or ball and eventual failure of the valve itself. There is also a chance of the ball vibrating inside the valve due to flow turbulence. The recess in the ball can also allow accumulation of material, which may not be acceptable in piping systems requiring a high state of purity.
German Utility Model DE-U 89 10 895 discloses a valve arrangement which is configured as a plastic lined ball valve. It has a one piece ball and shaft which passes through the housing, and is surrounded by a special lined packing gland, containing a spring loaded sealing element. This packing gland extends from the base of the ball-stem joint up to a certain height above the housing. This arrangement results in an additional joint to be sealed and an additional part to be lined with plastic material at a considerable cost.
There have been proposals for constructing valves having an integral ball and stem utilizing a center split body, such as shown in U.S. Pat. No. 4,696,323 (Neotecha) However, such an arrangement has the disadvantage in that, the body halves are sealed by a lined face/flange which is split at the shaft packing area. This is a potential cause for leakage as even the slightest misalignment of the two halves in the packing area will result in a gap between the packing and lining of the body. Also in this design the packing cover/gland used for loading the springs which compress the packing, itself acts as a mounting flange for the actuator. In this arrangement the screws used for fixing the mounting flange on the valve housing experience a shearing load due to the torque exerted by the actuator. The shearing effect can even cause failure of the screws or their loosening, leading to relaxation of pressure on the packing in, turn causing leakage thru the stem. Also if the mounting flange becomes loose accurate positioning of the ball is impossible.
In plastic lined ball valves the lining between the two body halves which acts as a seal, being plastic, tends to flow under compression or tends to leak due to uneven piping loads which exert a bending force on the valve body. This can result in leakage through the body halves, requiring frequent re-tightening of the body bolts. Additionally, the other seals within plastic valves, including the valve seats, must also be of a corrosion resistant or substantially inert material. Typically, such seals are formed from polytetrafluoroethylene, which is better known as PTFE. These plastic seats also tend to flow under compressive load and over time lose their effectiveness.
The mounting of an actuator on top of the ball valve without a mounting flange also requires additional brackets or fixtures to support the actuator. This means additional expenses in the form of parts and fasteners. The brackets/fixtures are required to be mounted either on the packing gland, or on top of the bonnet of the valve or on the main housing flanges, to fit the actuator. These brackets/fixtures increase the overall size of the valve and actuator assembly, causing space problems during erection of the pipeline. Also the large number of fasteners required to mount the actuator increases the chance of the actuator assembly becoming loose and leading to inaccurate control of the valve.
The integration of the actuator mounting flange into the body housing can eliminate all the problems related to actuator mounting, mentioned above. However, this requires the valve body to be side split and the stem to be long enough so as to project above the mounting flange. But the designs of heretofore existing plastic lined valves have failed to provide a valve with a one piece ball-stem and an integrated actuator mounting flange due to the difficulty of inserting and assembling a ball with an extra long stem into a side split body. A center split body cannot have an integrated actuator mounting flange.